A number of non-invasive methods of examining internal bodily organs, or sections of such organs, have become popular for diagnosing a variety of illnesses. One of these techniques is called Positron Emission Tomography (PET) or Positron Emission Transaxial Tomography (PET). In this method of developing internal bodily images, an array of sensors detects gamma rays emitted from tissues after the subject has been administered a natural biochemical substance (for example, gases, glucose or fatty acids) into which position-emitting radio-isotopes have been incorporated. A computer calculates the paths of the gamma rays (which result from collisions of positrons and electrons) and interprets the data to generate a tomographic image. The resultant tomogram represents local concentrations of the isotope-containing substance in the tissues. By proper choice of isotope-containing substance, various processes such as brain function, local blood flow, blood volume and other metabolic processes can be studied.
The short-lived radio-isotopes are administered by intravenous injection or by having the subject inhale a gas containing small quantities of the radio-isotope. Isotopes which are often incorporated into such gases or injections are carbon-11, nitrogen-13, oxygen-15 and fluorine-18. In present PET facilities, these radio-isotopes are derived from boron, carbon, nitrogen and neon targets, respectively, by bombarding the targets with high-energy (approximately 6-15 MV) protons or deuterons obtained from a particular accelerator.
Conventionally, the acelerator which is used to produce the isotope-generating particles is a cyclotron accelerator. However, cyclotron accelerators have significant drawbacks. Due to the short half-lives of the isotopes (ranging from approximately 2 minutes for oxygen-15 to 110 minutes for fluorine-18), the accelerator must be physically located in the medical center within a short distance from the PET scanning apparatus. At present, cyclotron accelerators suitable for use in a medical environment are expensive (on the order of 1-2 million dollars); large and heavy (15 to 20 tons) and require a trained staff to operate and maintain the apparatus.
In addition, the high-energy ions produced by the cyclotron accelerator are generally used to bombard gas targets to obtain the isotopes. Gas targets must be separated from the high vacuum of the accelerator by a metallic-foil window. Because the ion beam has high energy, the window is rapidly destroyed, thus increasing the costs of maintenance and requiring highly trained operators who must disassemble the device to replace the window.
These drawbacks have restricted the use of position emission tomography to large research hospitals that have sufficient space, staff and funding to support a conventional cyclotron accelerator facility.
While smaller, lower cost, low energy ion accelerators, such as tandem Van de Graff accelerators, have existed, it has not heretofore been considered possible to use such low energy accelerators to generate clinically significant quantities of medical isotopes and such accelerators have not been capable of generating anion beam with sufficient ion current to produce such isotopes.
Accordingly, it is an object of the present invention to provide an ion accelerator which can generate sufficient ion current in a low energy range for the production of clinically significant quantities of medical isotopes such as PET isotopes.
Another object of this invention is to provide an ion accelerator which can deliver higher currents than conventional low energy accelerators.
It is another object of the present invention to provide an ion accelerator which is smaller and lighter than existing cyclotron accelerators.
It is a further object of the present invention to provide an ion accelerator which costs less than conventional cyclotron accelerators.
It is yet another object of the present invention to provide an ion accelerator which can be easily operated by a single moderately-trained technician.
It is still a further object of the present invention to provide an ion accelerator which can operate with a solid target and thus does not require a metallic-foil window.
It is a further object of the present invention to provide an ion accelerator which has lower maintenance costs than conventional cyclotron accelerators.